The invention relates to a radiation detector, serving especially as a movement monitor, infrared detector reacting to thermal radiation, including a housing, and a window disposed in the housing for transmitting the radiation, having a size coordinated with the angle of coverage of optics disposed behind it, for focussing the radiation onto an optical sensor, preferably through a Fresnel lens.
Infrared detectors are used as movement monitors in zonal monitoring both inside and outside buildings. As passive detectors, they react directly to radiating objects which emit thermal radiation. An example of such a radiating object is also a person who intrudes into a zone to be monitored. There is consequently no need during monitoring for an additional transmitter such as is required with movement monitors of different types.
Whereas earlier, infrared detectors had only a relatively small, acute angle of coverage, one is known from EP-A2-0,113,468 which reaches an angle of coverage of up to 180.degree. in the horizontal. With this detector, the thermal radiation is focussed with the aid of collecting optics onto a sensor which is sensitive in the infrared band. The collecting optics consist of a multiplicity of mutually interconnected individual collector lenses, arranged in a semicircle around the sensor. In this way, each individual collector lens forms a strip-shaped segment of an axially segmented cylindrical section. Moreover, the collector lenses have the structure of a Fresnel lens, so that a wide coverage is guaranteed not only in a radial direction to the cylindrical collecting optics, but also axially along the strip-shaped collector lens.
Assuming that a detector of the type described is mounted on a wall so that the axis of the cylindrical collecting optics is vertically aligned, then, depending on its range, it can monitor the plane extending horizontally before it as far as the wall to which it is attached. For most applications, it is desired to have an angle of coverage so wide that it offers the possibility of an almost uninterrupted monitoring of the zone lying before it. However, problems arise if the range of the detector is too large, i.e. if it extends into a region in which permitted radiating objects are present. It is certainly true that generally an adjustment can be achieved by reducing the sensitivity of the detector, but for the very narrow sites, at which, for example, a road or footpath passes by, this measure often does not produce the desired result.
In order to avoid false reports due to an excessive angle of coverage, use has already been made in the past of the possibility of limiting the angle of coverage. This was done by sticking a matching plastic film, opaque in the infrared range to the Fresnel lens. As a rule, however, success is not achieved in the first attempt to restrict the angle of coverage so that detection remains confined to the desired region. It is therefore necessary to undertake a fitting process which necessitates repeatedly removing the film from the lens and once again sticking it on. In this connection, soiling of the lens and even, on occasion, lasting damage is almost unavoidable.
It is accordingly an object of the invention to provide a radiation detector, which overcomes the hereinafore- mentioned disadvantages of the heretofore-known devices of this general type and which is equipped with simply constructed means for facilitating a reduction of the coverage present without these means. In particular, the adjustment of the coverage is to be provided easily and without damaging the lens.